Plates and boundaries
- The Earth's crust is broken into tectonic plates, which float on the mantle and are moved by convection currents and the pull of sinking plates.
- Destructive boundaries: an oceanic plate sinks (subducts) under a continental plate, causing explosive volcanoes and earthquakes, as in the Andes.
- Constructive boundaries: plates move apart and magma rises to form new crust, with gentler volcanoes, as in Iceland on the Mid-Atlantic Ridge.
- Conservative boundaries: plates slide past each other, causing earthquakes but no volcanoes, as on the San Andreas Fault. Where continental plates collide, fold mountains form, such as the Himalayas.
- The Ring of Fire is a ring of volcanoes and earthquakes around the Pacific Ocean.
Earthquakes and volcanoes
- Earthquakes happen when pressure builds up where plates are stuck and is suddenly released. The focus is where the earthquake starts underground; the epicentre is the point on the surface above it. Seismometers measure the shaking.
- Composite volcanoes are steep, with layers of ash and lava and violent eruptions. Shield volcanoes have gentle slopes and runny lava.
- Volcanoes can be active, dormant (not erupted for a long time, but could erupt again) or extinct.
Effects and responses
- Primary effects happen straight away: buildings collapse and people are killed or injured. Secondary effects come later: fires, tsunamis, landslides, disease and homelessness.
- People live near volcanoes for fertile soil, geothermal energy, tourism and minerals.
- Reducing the risk: monitoring volcanoes, earthquake-proof buildings (cross-bracing and shock-absorbing foundations), hazard maps, drills and emergency kits.
Key terms
- Tectonic plate
- A large section of the Earth's crust.
- Convection current
- The circular movement of heat in the mantle that moves plates.
- Destructive boundary
- Where an oceanic plate sinks under a continental plate.
- Constructive boundary
- Where plates move apart and new crust forms.
- Conservative boundary
- Where plates slide past each other.
- Subduction
- One plate sinking beneath another.
- Focus
- The point underground where an earthquake starts.
- Epicentre
- The point on the surface directly above the focus.
- Seismometer
- An instrument that measures earthquakes.
- Composite volcano
- A steep volcano with explosive eruptions.
- Shield volcano
- A gently sloping volcano with runny lava.
- Dormant volcano
- A volcano that hasn't erupted for a long time but could erupt again.